1. Field of the Invention
The invention relates to a method for packet transmission using an ARQ protocol on transmission channels in a digital transmission system, in which, for channel coding, turbo coding is carried out. This turbo coding is carried out a turbo coder at the transmitter end and turbo decoding is carried out in a turbo decoder at the receiver end using soft-decision output signals, with a return channel, being provided, by the way of which the receiver re-quests the information from faulty packets.
2. Description of the Related Art
The use of turbo codes for digital transmission systems is investigated in P. Jung xe2x80x9cComparison of Turbo-Code Decoders Applied to Short Frame Transmission Systemsxe2x80x9d, IEEE Journal on Selected Areas in Communications, Volume 14 (1996) pages 530-537, with both coders and decoders being investigated for the turbo codes in the transmission path. Decoding of the turbo codes is based on the use of soft-input/soft-output decoders, which can be produced using either MAP (Maximum a-posteriori) symbol estimators or MAP sequence estimators, for example, an estimator using an a-priori soft-output Viterbi algorithm (APRI-SOVA). This publication describes four different decoder arrangements and their capabilities to process specific error rates. Furthermore, the performance of these decoders is investigated for different applications. It has been found that the turbo codes and their iterative decoding are an effective measure against packet errors.
ICC ""95, Seattle, Wash., Jun. 18-22, 1995, xe2x80x9cTurbo Codes for BCS Applicationsxe2x80x9d, D. Divsalar and F. Pollara, proposes turbo codes to achieve error correction virtually as far as the Shannon limit. The use of relatively simple component codes and large interleavers are considered for this purpose. In this publication, the turbo codes are produced in a coder using multiple codes, and are decoded in a suitable decoder. The turbo codes were introduced by Berrou et al. 1993 (see C. Berrou, A. Glavieux and P. Thitimayshima, xe2x80x9cNear Shannon limit area correction coding: Turbo codesxe2x80x9d Proc. 1993 IEE International conference on communications, pages 1064-1070). On the one hand, this method allows very good error correction to be achieved.
Turbo equalization is known from ETT European Transactions on Telecommunications, Vol. 6, No. 5, September-October 1995, xe2x80x9cIterative Correction of Intersymbol Interference: Turbo-Equalizationxe2x80x9d, Catherine Douillard et al., whose use is intended to overcome the disadvantageous effects of intersymbol interference in digital transmission systems which are protected by convolution codes. The receiver makes two successive soft-output decisions, which are made in an iterative process by a symbol detector and a channel decoder. Each iteration makes use of extrinsic information from the detector and the decoder for the next iteration, as with turbo decoding. Douillard, et al. found that intersymbol interference effects in multipath channels can be overcome by turbo equalization.
The publication xe2x80x9cA Novel ARQ Technique using the Turbo Coding Principlexe2x80x9d, Narayanan et al., IEEE Communications Letters, Volume 1, No. 2, March 1997, pages 49-51 describes an ARQ-III method using punctured turbo codes, in which the bits which were punctured for the first transmission are transmitted after the occurrence of faulty data packets. The receiver then combines the punctured code and the punctured bits, and thus obtains the unpunctured code.
Future transmission systems, for example the European UMTS (Universal Mobile Telecommunications System), require the support of a large number of co-existing carrier services with carrier data rates of up to 2 Mbit/s in a flexible manner, with the best-possible spectral efficiency being desirable. An MA (Multiple Access) scheme has been developed in the ACTS (Advanced Communications Technologies and Services) project AC090 FRAMES (Future Radio Wideband Multiple Access Systems), which is called FRAMES Multiple Access (FMA) and satisfies the UMTS requirements. As a third-generation transmission system, which covers a wide range of application areas, carrier services and widely differing scenarios, FMA must to comply with present and future developments of UMTS radio interface standards. FMA comprises two operating modes, namely WB-TDMA (Wideband Time Division Multiple Access) with and without spreading and compatibility with GSM (Global System for Mobile Communications) and WB-CDMA (Wideband Code Division Multiple Access). Although, essentially, a system based on FMA is considered here, it is also possible to include other transmission systems using multiple access methods, for example FDMA (Frequency Division Multiple Access) MC-CDMA (Multicarrier-CDMA) or combinations of the said transmission systems.
With regard to the high performance of turbo codes, it is desirable to use these codes in digital transmission systems. The complex requirements, for example for FMA, mean, however, that it is necessary when using such turbo codes to ensure that the data transmission makes full use of the capabilities of the turbo codes.
The invention is based on the object of providing a method for packet transmission using an ARQ protocol on transmission channels in a digital transmission system in which turbo coding is used for channel coding, in which the channel loading from ARQ can be kept as low as possible by way of a new turbo code and puncturing matched to it.
The present invention implements a method for packet transmission using an ARQ (Automatic Repeat ReQuest) protocol on transmission channels in a digital transmission system, comprising the steps of turbo coding, for channel coding, in a turbo coder at a transmitter end, utilizing a punctured turbo code with a variable coding rate, wherein said coding rate is chosen as a function of a Quality of Service of a transmission channel which is one of the transmission channels, turbo decoding in a turbo decoder at a receiver end, requesting coded packets incorrectly sent, by the receiver via a return channel, transmitting a portion of information suppressed by a puncturing of turbo code in a previous transmission, constituting additionally transmitted information, when an incorrectly coded packet is re-transmitted, inserting said additionally transmitted information into already existing information at said receiver end, and decoding resultant completed information again.
When the RCPTC is used, the coding rate can be set by suitable puncturing of the systematic or non-systematic information at the output of the turbo encoder. An increase in the coding rate, i.e., more information being punctured, in this case means that the decoding result is worse for a given channel quality. This means that the bit error rate BER increases. The use of the RCPTC for channel coding means that it is not necessary to transmit the entire packet once again in packet-switching services when an ARQ is initiated. The first transmission of the packet is carried out using a high coding rate, with little error protection. If the packet is identified as being faulty, then an ARQ is initiated. After this, rather than transmitting the entire packet again, only the information which was punctured in the first transmission, or a portion of this punctured information, is transmitted. The coding rate is thus compatibly matched to the channel as a result of which, overall, less data need be transmitted over the channel. The advantage of this method is thus that the total load on the channel is reduced.
In this document, the term Quality of Service is used as follows. Specific QoS criteria (QoS=Quality of Service) apply to various services, and the definitions of the QoS criteria for various carrier services have been worked out in the course of FRAMES. One important component of a QoS criterion is the carrier data rate R. The QoS criterion also includes a maximum permissible error rate PbG or a packet loss rate PlG in conjunction with a maximum failure probability PoutG. In the case of line-switched services, the probability Pr{Pb greater than PbG} of the instantaneous bit error rate Pb exceeding PbG must not be greater than PoutG, i.e.,
Pr{Pb greater than PbG} less than PoutG.
For voice transmission, PbG is equal to 10xe2x88x923 and PoutG is equal to 0.05. A similar condition for the instantaneous packet loss rate Pl applies to packet services:
Pr{Pl greater than PlG} less than PoutG.
Apart from the criteria relating to Pr, there are also other conditions relating to the QoS criterion. However, the QoS parameters PbG, PlG and PoutG will mainly be considered here, which relate directly to the choice of the error correction code (ECC). For ECC, the coding rate Rc is essentially governed by the multiple access method, the modulation and the packet parameters. In other words, the coding rate Rc is directly related to the question as to whether a QoS criterion is or is not satisfied for a specific service.
In a method in which a soft-input/soft-output symbol or sequence estimator is used at the receiver end, it is advantageous for the Quality of Service to be determined from the variances "sgr"2 of the soft-decision output signals from the turbo decoder; the bit error rate is advantageously calculated from the variances "sgr"2 as a measure of the Quality of Service.
In a method in which an MAP symbol estimator or an MAP sequence estimator is used at the receiver end, it is advantageous for the Quality of Service to be determined from the variances "sgr"2LLR of the soft-decision output signals from the turbo decoder.
In a method in which a Viterbi algorithm for sequence estimation is used at the receiver end, it is advantageous for the Quality of Service to be determined from the variances "sgr"2VIT of the soft-decision output signals from the turbo decoder.
Since the method according to the invention can be used not only with MAP estimators but also for estimation using a Viterbi algorithm, there is virtually no limitation with regard to the most important methods for sequence and symbol estimation. This is true even though this statement is used only in conjunction with an MAP symbol estimator in the following specific description.
According to one advantageous refinement, the method according to the invention is characterized in that Berrou""s puncturing is used for puncturing, in which only the non-systematic information is punctured. This type of puncturing is advantageous for relatively low signal-to-noise ratio values.
According to one advantageous refinement, the method according to the invention is characterized in that UKL puncturing is used for puncturing, in which both systematic information and non-systematic information are punctured. This type of puncturing is advantageous for relatively high signal-to-noise ratios, and thus for bit error rates of  less than 10xe2x88x924.
According to one advantageous refinement, the method according to the invention is characterized in that when the repeat transmission is made, the only bits which are transmitted are those which are additionally available at the next lower coding rate, since they are not punctured. In consequence, only the minimum required information is transmitted in order to achieve the aim of reasonable transmission quality.
According to one advantageous refinement, the method according to the invention is characterized in that the method is repeated until the packet is decoded without any errors or all of the coded information in a packet is transmitted. The entire potential of the coding is thus always fully utilized to correct the errors in a packet.